When selecting a wind turbine for a given operating location, consideration is given to characteristics of the site such as the complexities of the site terrain and the average wind conditions. The turbines chosen can ideally operate at rated power for as much of the time as possible. However, in practice, wind speeds are variable and the turbine must be able to cope with a wide variety of wind speeds. At lower wind speeds the power output will either be zero, if there is negligible wind, or below rated power. Once the wind speed increases to above that required for generation of rated power, the turbine will protect itself from damage, for example, by varying the pitch of the blade to reduce the power extracted from the wind. In extreme cases the turbine may shut down or yaw out of the wind to prevent catastrophic damage.
International Standard IEC 64100-1, 3rd Edition, defines normal and extreme wind conditions which a wind turbine must be designed to withstand. These requirements place limitations on the design of wind turbine components as all components must be able to withstand the most extreme conditions.
We have appreciated that there are certain wind conditions which, although are not recognised as extreme under the IEC 64100-1 definition, are nevertheless potentially damaging. Extreme conditions tend to cause serious damage through a single event, such as an extreme gust or change of direction with high wind speed. However, we have appreciated that there is a class of events which cause loading on the turbine components of a magnitude which is lower than the design load, but high enough that only a small number of such loads or load cycles is needed to cause failure of a component. These loadings, referred to as low cycle fatigue loads, may require fewer than 100 instances of loading or loading cycles for failure to occur. As wind turbines are designed for a lifetime of twenty years, this type of event need only occur a handful of times a year for failure to take place during the lifetime of the wind turbine. In the past it has not been possible to assess the rate of accumulation of low cycle fatigue in wind turbine components.
An example of a condition which causes a low cycle fatigue is a combination of high wind speed, high turbulence and a high standard deviation of wind direction. We have further appreciated that a reduction in exposure to events causing low cycle fatigue will remove some of the design constraints on wind turbine components. Not only will this preserve and extend the lifetime of the components but it will also enable blades to be made using less material, and therefore more cheaply. Alternatively larger blades may be used at a given site enabling greater energy extraction from the wind.